The use of roughened emitting surfaces to enhance LED extraction efficiency is a common aspect of numerous LED designs. Roughening may be applied to different types of LED structures including InGaN, AlInGaP systems, and in flip chip bonded, and vertical thin film device architectures, and others.
U.S. Pat. No. 7,875,533, “PACKAGE INTEGRATED THIN FILM LED, AND DEVICES”, issued to John Epler, Paul Martin and Michael Krames on 25 Jan. 2011, and incorporated by reference herein, discloses roughening of the GaN light emitting surface of an LED to enhance the light extraction efficiency using a photo-electrochemical etch process using a KOH solution. The depth of the etching is controlled using an etch stop layer that is grown during the formation of the light emitting device. In like manner, USPA 2010/0025717, USPA 2009/0146170, USPA 2008/0113463, and U.S. Pat. No. 7,749,782 also disclose techniques for improving light extraction efficiency by roughening the light emitting surface, and are incorporated by reference herein.
Each of the above referenced methods creates a substantially uniformly roughened surface that allows for maximum light extraction, often allowing for the extraction of twice as much light as the original unroughened surface. With continuing increases in light emission efficiency, a doubling of the light extraction efficiency may be undesirable in certain applications. For example, it may be desirable to limit the total light output to a customer's maximum-flux specification, to conform to a particular standard, or to achieve a particular lighting effect.
For each roughening method it may be possible to change the light extraction efficiency by changing the characteristics of the roughening, such as by changing the parameters of the roughening process to increase or reduce the coarseness or other aspect of the resultant roughened surface. However, the cost of developing customized processes may be excessive, and the achievable range of controllable efficiency may be limited or subject to variances in the process.